Ion chromatography system using catalytic gas elimination

ABSTRACT

A liquid chromatographic system is provided including catalytically combining hydrogen and oxygen gases in the chromatography eluent stream in a catalytic gas elimination chamber, to form water and thereby reduce the gas content in the eluent stream. Also, a liquid ion chromatographic system in which the effluent from the detector is recycled to a membrane suppressor and then is mixed with a source of eluent for recycle to the chromatographic column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of pending U.S. patentapplication Ser. No. 11/065,335, filed on Feb. 23, 2005.

BACKGROUND OF THE INVENTION

Since it was introduced in 1975, ion chromatography has become a widelyused analytical technique for the determination of anionic and cationicanalytes in various sample matrices. In ion chromatography, dilutesolutions of acids, bases, or salts are commonly used as theelectrolytes in chromatographic eluents. Traditionally,

These eluents are prepared off-line by dilution with reagent-gradechemicals. Off-line preparation of chromatographic eluents can betedious and prone to operator errors, and often introduces contaminants.For example, dilute NaOH solutions, widely used as the electrolytes ineluents in the ion chromatographic separation of anions, are easilycontaminated by carbonate. The preparation of carbonate-free NaOHeluents is difficult because carbonate can be introduced as an impurityfrom the reagents or by adsorption of carbon dioxide from air. Thepresence of carbonate in NaOH eluents often compromises the performanceof an ion chromatographic method, and can cause an undesirablechromatographic baseline drift during the hydroxide gradient and evenirreproducible retention times of target analytes. Therefore, there is ageneral need for convenient sources of high purity acid, base, or saltfor use as eluents in the ion chromatographic separations.

A number of approaches that utilize the electrolysis of water andcharge-selective electromigration of ions through ion-exchange mediahave been investigated by researchers to purify or generate high-purityion chromatographic eluents.

U.S. Pat. No. 5,045,204 describes that an impure acid or base ispurified in an eluent generator while flowing through a source channelalong a permselective ion exchange membrane which separates the sourcechannel from a product channel. The membrane allows selective passage ofcations or anions. An electrical potential is applied between the sourcechannel and the product channel so that the anions or cations of theacid or base pass from the former to the latter to generate therein abase or acid with electrolytically generated hydroxide ions or hydroniumions, respectively. This system requires an aqueous stream of acid orbase as a starting source or reservoir.

U.S. Pat. Nos. 6,036,921, 6,225,129, 6,316,271, 6,316,270, 6,315,954,and 6,682,701 describe electrolytic devices that can be used to generatehigh purity acid and base solutions by using water as the carrier. Usingthese devices, high purity, contaminant-free acid or base solutions areautomatically generated on-line for use as eluents in chromatographicseparations. These devices simplify gradient separations that can now beperformed using electrical current gradients with minimal delay insteadof using a conventional mechanical gradient pump. The use of high-purityeluents generated by the electrolytic eluent generators cansignificantly improve the performance of ion chromatographic methods.

United States Patent Application No. 2004/0048389 describes electrolyticdevices for generating salt solutions. In these devices, an acid or baseis generated in an aqueous solution by the steps of: (a) providing asource of first ions adjacent an aqueous liquid in a first acid or basegeneration zone, separated by a first barrier (e.g., anion exchangemembrane) substantially preventing liquid flow and transporting ionsonly of the same charge as said first ions, (b) providing a source ofsecond ions of opposite charge adjacent an aqueous liquid in a secondacid or base generation zone, separated by a second barrier transportingions only of the same charge as the second ions, and (c) transportingions across the first barrier by applying an electrical potentialthrough said first and second zones to generate an acid-containingaqueous solution in one of said first or second zones and abase-containing aqueous solution in the other one which may be combinedto form a salt solution such as a solution of potassium carbonate. Theadvantages of using these electrolytic devices for eluent generation inion chromatography were demonstrated.

The continuous operation of an ion chromatography system can consume asignificant amount of eluents. The consistent preparation of such largeamount of the eluent as well as the disposal of the used eluent can poseserious logistical challenges to the system operators in terms of costsand labor, especially in cases where unattended or less frequentlyattended operations are required. Even though it overcomes a number ofissues associated conventional approaches of eluent preparation in ionchromatography, the use of on-line electrolytic eluent generationdevices still requires a constant supply of high purity water from anexternal source for continuous operation and waste disposal issueremains.

To simplify ion chromatographic operations, minimize waste disposal, andreduce operating costs, it would be advantageous to provide a systemcapable of recycling prepared eluent or water used in the electrolyticeluent preparation.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a chromatographic method is providedincluding the steps of (a) injecting sample ionic species into anaqueous eluent stream, (b) chromatographically separating said sampleionic species in said eluent stream by flowing the same throughchromatographic separation medium, (c) detecting said separated sampleionic species in said eluent stream effluent from said chromatographicmedium; and (d) catalytically combining hydrogen and oxygen gases, orcatalytically decomposing hydrogen peroxide, or both, in said eluentstream in a catalytic gas elimination chamber, to form water and reducethe gas content in said eluent stream.

In another embodiment, a chromatographic method is provided comprisingthe steps of (a) chromatographically separating sample ionic species inan aqueous liquid eluent stream flowing through a chromatographyseparation medium, to form a chromatography effluent, (b) suppressingthe chromatography effluent from step (a) by flowing it through achromatography effluent flow channel on one side of a first ion exchangemembrane in a membrane suppressor, (c) flowing suppressor effluent fromthe chromatography effluent flow channel past a flow-through detector toform a detector effluent stream, (d) recycling the detector effluentfrom step (c) to a detector effluent flow channel in the membranesuppressor on the opposite side of the first membrane from thechromatography effluent flow channel, and (e) mixing effluent from thedetector effluent flow channel with a source of eluent and flowing themixture to the chromatography separation medium.

In other embodiments of the invention, apparatus is provided capable ofperforming the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 are schematic representations of apparatus according todifferent embodiments of the present invention.

FIGS. 4 and 5 are chromatograms illustrating experimental results usingthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The system of the present invention is useful for determining ionicspecies which are solely anions or cations. Suitable liquid samplesinclude surface waters, other liquids such as industrial chemical waste,body fluids, beverages or drinking water. The term “ionic species”includes molecular species in ionic form and molecules which areionizable under the conditions of the present invention. The term“eluent” refers to the solution flowing in a liquid chromatographysystem which carries a sample to be detected. At times herein, the termeluent also refers to the electrolyte in that solution. The eluentnormally is water-based but can include an organic solvent so long as itis electrochemically stable.

In certain embodiments, the invention includes a suppressor. The purposeof a suppressor is to reduce the conductivity and noise of the analysisstream background while enhancing the conductivity of the analytes(i.e., increasing the signals/noise ratio), while maintainingchromatographic efficiency.

Referring to FIG. 1, a simplified apparatus for performing the presentinvention is illustrated. This system includes a suppressor with recycleof the effluent from a detector to the suppressor. That portion of thesystem is similar to the general system illustrated in U.S. Pat. No.5,248,426. One important difference of the system of FIG. 1 from thatpatent is that the effluent from the suppressor is recycled to theseparation column, preferably after mixed with an eluent in an effluentreservoir, as part of the eluent electrolyte used for separation. In apreferred embodiment, one or more eluent purification columns such asion trap columns are used in the system to remove contaminants,typically in ionic form, in the recycled stream prior to use as aneluent in the separation column.

Referring specifically to FIG. 1, the system includes chromatographicseparation medium, typically in the form of chromatographic separationmedium chromatographic column 10. (As used herein, the term “column”refers to a flow-through housing with an interior chamber in anyconfiguration for performing the indicated function.) Any knownchromatographic separation medium may be employed including ion exchangeresin, a porous hydrophobic chromatographic resin permanently attachedto ion exchange sites, and medium used for mobile phase ionchromatography (MPIC).

Arranged in series with column 10 is a suppressor 12 serving to suppressthe conductivity of the electrolyte of the eluent from column 10 but notthe conductivity of the separated ions. The effluent from suppressor 12is directed to a detector, preferably in the form of a detectorflow-through conductivity cell 14, for detecting the ion speciesresolved in column 10. A suitable sample including ion species issupplied through sample injection valve or injector 16 which is passedthrough the apparatus in the solution of eluent from an eluent source orreservoir 18 drawn by pump 20 which then passes through injection valve16. The chromatography effluent solution leaving column 10 is directedthrough suppressor 12 wherein the electrolyte is converted to a weaklyconducting form. The chromatography effluent from suppressor 12 passesthrough detector 14, schematically illustrated as a conductivity cell,in which the presence of ionic species produces an electrical signalproportional to the amount of ionic material. Such a signal is typicallydirected from detector 14 to a conductivity meter (not shown). Any otherknown detector useful for detecting ionic species in a chromatographysystem may also be employed including absorbance and electrochemicaldetectors.

In one embodiment, the effluent from conductivity detector 14, referredto as the detector effluent, is directed in a recycle conduit 22 to atleast one flow-through detector effluent flow channel 24 in suppressor12. An ion exchange membrane 26 separates detector effluent flow channel24 from chromatographic separation effluent flow channel 28 whichreceives the effluent from chromatography column 10. In the simplifiedversion illustrated, only a single detector effluent flow channel 24 isused. The system of the present invention is also applicable to othermembrane suppressors such as the sandwich suppressor type illustrated inU.S. Pat. No. 5,248,426. In a sandwich suppressor, the chromatographicseparation effluent flows through a central flow channel flanked by twodetector effluent flow channels separated by ion exchange membranes. Inthis embodiment, the detector effluent flow channels may be suppliedwith the detector effluent from conductivity detector 14 by use of asplitter valve. The details of such a sandwich suppressor and the use ofrecycle from the conductivity cell are supplied by the detector effluentflow channel as illustrated in U.S. Pat. No. 5,248,426. As illustrated,for anion analysis, the detector effluent flow channel is positivelycharged and hydronium ions are generated for passage through membrane 26according to the following equation:6H₂O→4H₃O+O₂+4e ⁻  (1)

In the chromatography effluent flow channel, cations of the electrolyte,e.g., sodium ions, pass through membrane 26 into the effluent of thecathode for electrolytic suppressor. Hydroxide is converted to wateraccording to the following equation:OH⁻+H3O⁺→2H₂O  (2)

In a preferred embodiment, the suppressor is of the electrolytic type asillustrated in FIGS. 3 and 4 of U.S. Pat. No. 5,352,360.

Suitable eluent solutions for anion ion chromatography include alkalihydroxides, such as sodium hydroxide, alkali carbonates andbicarbonates, such as sodium carbonate, alkali borates, such as sodiumborate, combinations of the above, and the eluent systems of theaforementioned patents.

The recycle system of the present invention is also applicable to theanalysis of cations (e.g., lithium, sodium, ammonium, potassium,magnesium, and calcium). In this instance, the electrolyte of the eluentis typically an acid which does not damage the membrane. Methanesulfonic acid has been found to be inert to the membrane underelectrolytic conditions. Other acids such as nitric acid andhydrochloric acid produce electrochemical by-products that may damagethe membrane and are, thus, not generally preferred for that typicalmembrane.

In the effluent recycle system of U.S. Pat. No. 5,248,426, the effluentfrom the detector effluent flow channel is directed to waste. Incontrast, in the system of FIG. 1, the effluent is redirected to theseparation column 10, preferably by flow through the eluent reservoir18.

Referring specifically to FIG. 1, the effluent from the detectoreffluent channel flows in line 30 through optional catalytic gaselimination column 31 and optional eluent purification column 32 andfrom there through tubing projecting through a closure 36 of container38 of eluent reservoir 18. An optional gas vent 40 is provided inreservoir container 38 to vent hydrogen and oxygen gases which aregenerated electrolytically in the system. Eluent solution from reservoir18 is directed in line 42 to separation column 10 as the source ofeluent for separation. As illustrated, the eluent in line 42 flowsthrough pump 10 and optional eluent purification column 44 prior toseparation column 10.

Also as illustrated in FIG. 1, optional ion trap column 23 can be placedin line 22 between conductivity cell 14 and suppressor 12. Preferably,the ion trap column 23 is packed with anion exchange resin in carbonateform for anion analysis using carbonate eluents or anion ion exchangeresin in the hydroxide form for anion analysis using hydroxide eluents.Typically, it only removes ions of one charge, positive or negative. Forcation analysis using acid eluents, the ion trap column may be packedwith cation exchange resin in the hydronium form. The ion trap columnserves the function for retaining analyte ions in the suppressed eluent.

For a system in which eluent is recycled from the detector to theseparation column, it is preferable to remove contaminants of sampleinjected through injection valve 16, and other trace contaminants may begenerated from operation of the ion chromatography system. This can beperformed by using one or both of purification columns 32 or 44. Oneform of eluent purification column preferably includes an inlet sectionand an outlet section, not shown, with a strongly acidic cation exchangematerial, e.g., resin in the inlet section, preferably in the form ofthe cation of the eluent used. For example, the resin preferably in theform of sodium for a sodium carbonate eluent or in the hydronium formfor a sulfuric acid eluent. The outlet section may be packed withstrongly basic cation exchange material, e.g., resin in the form of theanion of the flowing eluent. For example, the form may be carbonate forthe sodium carbonate eluent or sulfate for the sulfuric acid eluent. Itis preferable to use highly cross-linked and macroporous high area forboth the cation exchange resin and the anion exchange resin used inpurification columns. It is preferable to use resins havingcross-linking of at least 20%, preferably at least 30% and surface areaof at least 10 m²/g, preferably at least 20 m²/g. Such resins areeffective in removing components of the injected sample and other tracecontaminants generated in the system. Examples of such resins include AGMP-50 strongly acidic cation exchange resin and AG MP-1M strongly basicanion exchange resin available from Bio-Rad (Hercules, Calif.). Otherforms of optional eluent purifiers may be employed.

Optionally, the eluent purification columns 32 or 44 may also include anadditional section of neutralized porous resin, preferably of highsurface area to remove non-charged contaminants in the recycle eluent.In the illustrated embodiment, a larger eluent purification column 32 isplaced upstream of suppressor 12 in comparison to the optional smallerpurification column 44 placed at the outlet of pump 20 to further purifythe eluent prior to entering separation column 10. If the secondpurification column 44 is used, it preferably has about 50% or less ofthe capacity of the purification column 32. Suitably, column 32 has anion exchange capacity of at least 0.5 milliequivalents.

In general, the eluent purification column and the ion trap columnremove the ions from the sample injected and some trace componentsderived at the system. When the purified eluent is recycled back intoeluent reservoir 18, the solution typically contains a mixture ofelectrolytically generated hydrogen gas and oxygen gas. Some of thesegases may be removed from the eluent reservoir 18 equipped through a gasvent port 36.

In a preferred embodiment, a catalytic gas elimination column 31,including an internal flow-through chamber, is used to remove hydrogenand oxygen gases in an eluent chromatography system which recycleseluent from a detector for reuse. In one embodiment, column 31 isprovided in line 30 or 34 before or after column 32.

It is useful to discuss the principles of operation of a catalytic gaselimination chamber typically contained in a flow-through housing termeda column herein. Even though the combination of hydrogen and oxygen toform water is an exothermic process, hydrogen and oxygen do not reactautomatically when mixed together. The reason for this is the relativelylarge activation energy needed to begin the reaction. The mechanism issomewhat complex. It is a free radical mechanism with one of theinitiation steps is:H₂(g)→2H·(g)  (3)

Breaking the bond between the two hydrogen atoms requires 432 kJ/mole.This energy can be initially provided by a spark or a flame. After thereaction begins, the produced energy provides the necessary energy tocontinue breaking apart the hydrogen molecules. A catalyst provides analternative mechanism that has a lower activation energy, this allowsthe reaction to proceed without the requirement of the initial additionof energy such as a flame or spark.

Platinum is known to catalyze the reaction of hydrogen and oxygen.(Nature, vol. 390, 495-497, 1997; Journal of Chemical Physics, vol. 107,6443-6447, 1997; Surface Science, vol. 324, 185-201, 1995.) If platinumis placed into a container filled with hydrogen and oxygen, the platinumbegins to glow as it heats up, and water droplets condense in thecontainer. Cooling the platinum so that it doesn't just ignite themixture provides a smooth conversion of the hydrogen and oxygen towater. This reaction occurs because platinum provide a new route for thereaction. In this new pathway, hydrogen molecules react with theplatinum atoms on the surface of platinum. This reaction breaks the H—Hbond and forms two Pt—H bonds. The energy of activation for this processis small. Oxygen reacts with these Pt—H groups to form water, again,with a very small energy of activation. Through this low energy routefor the reaction, platinum catalyzes the recombination of oxygen andhydrogen gases. The term “catalyst” encompasses any catalyst thatperforms this function. Platinum is described as one specific example ofsuch a catalyst.

The above principles of using the known property of a catalyst such asplatinum to catalytically induce the reaction between hydrogen gas andoxygen gas to form water provide the basis for the catalytic gaselimination chamber of the present invention for use in liquidchromatography. This permits the elimination of these gaseouselectrolysis byproducts of an electrolytic suppressor in the eluent in asystem like FIG. 1. In such a system, the effluent from the outlet ofthe electrolytic suppressor regenerant chamber 24 is passed through thecatalytic gas elimination column 31, where the hydrogen and oxygenpresent in the effluent react catalytically to form water. Column 31 maybe packed with a suitable catalyst such as pure Pt in metal particle,mesh, or foil form. Alternatively, the column may also be packed withother inert substrates coated with Pt. In the present invention,optional catalytic gas elimination column 31 serves several importantfunctions. First, it provides an elegant means to conveniently eliminatethe build up hydrogen and oxygen gases and thus facilitates theoperation of continuous eluent recycle. Second, the water-formingreaction of hydrogen and oxygen is expected to be stoichiometric in thecolumn, and the amount of water formed is expected to be in principlethe same as the amount of water that is originally consumed to producehydrogen and oxygen gases in the electrolytic operation of thesuppressor. In principle, this feature eliminates an increase in theconcentration of eluent due to the consumption of water in theelectrolytic operation of the suppressor. Third, the Pt catalytic gaselimination column also serves the function to catalytically decomposetrace levels of hydrogen peroxide which may be formed during theelectrolytic operation of the suppressor. The presence of hydrogenperoxide in the recycled eluent is potentially detrimental since it mayattack the ion exchange functional groups in the separation column andthe eluent purification column in the system and degrade the columnperformance. The use of Pt to catalyze the decomposition of hydrogenperoxide is well known (Bull. Korean Chem. Society, 1999, vol. 20(6),696; U.S. Pat. No. 6,228,333).

The catalytic gas elimination chamber may have a wide variety ofphysical forms. One embodiment uses a chromatographic column housingwith fritted flow-through end fittings that are used to retain eitherpure Pt metal particles, mesh, or foils packed inside the column. Thecolumn may also be packed with other inert substrates coated with Pt. Ina preferred embodiment, the internal diameter of the column is 0.1 mm orlarger and the length of the column is 0.5 cm or longer. It is preferredto use Pt packing material in forms that provide high surface area toincrease its catalytic efficiency. It is also preferred to operate thecatalytic gas elimination chamber in flow rates ranging from 0.1 uL/minto 50 mL/min although other flow rates may be used.

A number of systems are known for practicing eluent recycling inchromatography systems and ion chromatography systems. U.S. Pat. No.6,027,643 discloses a method and apparatus for electrolyticallygenerating an acid or base in an aqueous solution and for simultaneouslysuppressing conductivity of the eluent in an ion exchange bed afterchromatographic separation in an ion chromatography system. U.S. Pat.No. 6,562,628 teaches a combination of electrolytic suppressor andseparate eluent generator and method for use such devices in an ionchromatography system. U.S. Pat. Nos. 6,093,327 and 6,558,551 describeseveral different embodiments of electrolytic devices and their uses inion chromatography. Since deionized water is used as the preferredcarrier stream in some embodiments of ion chromatography systemsdisclosed, the above patents describe that the spent water can berecycled after passing through an ion exchange water polisher column.However, these patents do not address the potential problems associatedwith the presence of hydrogen and oxygen gases in the spent stream ofthe water to be recycled.

Referring specifically to FIG. 2, an ion-reflux based chromatographysystem using water recycle is illustrated using the principles and thesame system as FIG. 1 of U.S. Pat. No. 6,562,628 with certaindifferences. A catalytic gas elimination column, an eluent reservoir,and an eluent purification column have been added. A pump for the eluentfrom the reservoir is illustrated. Further, optional degaser 56 is notshown. The system first will be briefly described with respect to thecommon elements of FIG. 1 of the '628 patent and FIG. 2 herein, followedby a description of the added elements.

The system of FIG. 2 illustrates improved methods to recycle water in anion chromatography system through the use of a catalytic gas eliminationcolumn, preferably in combination with water purification columns. Itillustrates the combined use of water purification columns and thecatalytic gas elimination column for recycling water in an ion-refluxbased ion chromatography system that generates and recycles potassiumhydroxide eluents for anion analysis. The detailed functions of theelectrolytic eluent generation and recycle module and electrolyticsuppressor have been disclosed in the '628 patent. In this ionchromatography system, the deionized water is used as the preferredcarrier stream in the electrolytic generation and recycle of potassiumhydroxide eluent. The effluent from the outlet of the eluent generationand recycle module is a mixture of water, hydrogen gas, oxygen gas, andpossibly some trace components derived from the operations of the entireion-reflux based ion chromatography system. To recycle the water, theeffluent from the eluent generation and recycle module is first passedthrough the catalytic gas elimination column to eliminate hydrogen andoxygen gases. Since stoichiometric amounts of hydrogen and oxygen gasesare generated in the electrochemical processes occurring in theelectrolytic eluent generation and recycle module and the electrolyticsuppressors, it is expected that hydrogen and oxygen combine to formwater in the same amount that is originally consumed. The effluent fromthe catalytic gas elimination column is then passed through the waterpurification column to remove the remaining trace ionic and nonioniccontaminants. The purified water is routed back to the water reservoir.

The additional components in FIG. 2 herein compared to FIG. 1 of the'628 patent are similar in structure and function to the correspondingcomponents in the embodiment of FIG. 1 herein. Thus, like parts will bedesignated with like numbers and the descriptions of such componentswill apply.

Referring specifically to FIG. 2, the solution leaving the eluentgenerator in line 165 flows through catalytic gas elimination column 31,through an eluent purification column 32 (corresponding to polishingcolumn 67 in FIG. 1 of the '628 patent) through line 34 into container38 for eluent reservoir 18. The recycled solution in line 34 is mixed ineluent reservoir 18 and is directed via pump 20 in line 42 through asecond eluent purification column 44. (A vent pipe in reservoir 18 isnot illustrated because hydrogen and oxygen gases may be eliminated incolumn 31.) From there on, the system is as described above.

In a similar manner, a catalytic gas elimination column may beadvantageous used in any other chromatography system, such as referredto above, in which the detector effluent is recycled as a source ofeluent liquid.

Referring again to FIG. 2, an ion chromatography system is illustratedusing a continuous electrolytically regenerated packed bed suppressor(CERPBS) form of suppressor and one embodiment of the eluent generator.The system includes an analytical pump 20 connected by tubing 112 tosample injection valve 114 which in turn is connected by tubing 116 to aflow-through chromatographic separator 118 typically in the form of achromatographic column packed with chromatographic resin particles. Theeffluent from chromatographic column 118 flows through tubing 120 to apacked ion exchange resin bed flow-through suppressor 122. Typically,suppressor 122 is formed of a column 124 packed with an ion exchangeresin bed 126 of the type used for ion chromatography suppression.Electrodes are spaced apart in the suppressor, with at least oneelectrode separated from the resin by a barrier described below. Theelectrodes are connected to a direct current power supply, not shown.The configuration is such that with an aqueous stream flowing throughthe suppressor and the application of power, water in the aqueous streamis electrolyzed to form a source of hydronium ion or hydroxide ion tocontinuously regenerate the ion exchange resin bed during the analysis.

The suppressor effluent is directed through tubing 130 to a suitabledetector 132 and then eventually to waste. A preferred detector is aconductivity detector with a flow-through conductivity cell. Thechromatography effluent flows through the cell.

Suppressor 122 generates hydronium ions (and oxygen gas) at the anodeand hydroxide ions (and hydrogen gas) at the cathode. That is, awater-containing eluent solution including electrolyte is directed fromthe pump and through tubing 112. Sample is injected through sampleinjection valve 114, and is directed by tubing 116 into chromatographiccolumn 118 to form a first chromatography effluent including separatedionic species of the sample. For simplicity of description, unlessotherwise specified the system will be described with respect to theanalysis of anions using an eluent solution including sodium hydroxideas the electrolyte.

A suitable sample is supplied through sample injection valve 114 whichis carried in a solution of eluent supplied from pump 20. Anode 136 isdisposed at the outlet end of resin bed 126 in intimate contact with theresin therein. The effluent from bed 126 is directed to a detectorsuitably in the form of a flow-through conductivity cell 132 of theconductivity detector (not shown), for detecting the resolved anions inthe effluent, connected to a conductivity meter.

In the detector, discussed regarding FIG. 1, the presence of anionsproduces an electrical signal proportional to the amount of ionicmaterial.

The system also includes an optional component for pressurizing theeffluent from suppressor 122 prior to detection to minimize adverseeffect of gases (hydrogen or oxygen) generated in the system as will bedescribed hereinafter. As illustrated in FIG. 1, such pressurizing meanscomprises a pressure restrictor 138 downstream of conductivity cell 132to maintain the ion chromatography system under pressure.

Column 124 is typically formed of plastic conventionally used for an ionexchange column. It has a cylindrical cavity of a suitable length, e.g.,60 mm long and 4 mm in diameter. It is packed with a high capacitycation exchange resin, e.g., of the sulfonated polystyrene type. Theresin is suitably contained in the column by a porous frit which servesto provide an outlet to the column. In the illustrated embodiment, theporous frit is porous electrode 136 which serves the dual function ofcontainment of the resin and as an electrode.

A barrier 140 separates bed 126 from electrode 142 in the interior of ahollow housing defining an ion receiving flow channel in electrodechamber 144 preventing any significant liquid flow but permittingtransport of ions only of the same charge as the charge of exchangeableions on resin bed 126. For anion analysis, barrier 140 is suitably inthe form of a cation exchange membrane or plug separating electrodechamber 144 from the cation exchange resin.

A conduit 148 is provided to direct the aqueous liquid stream to theinlet 150 of electrode chamber 144. Conduit 152 takes the effluent fromchamber 144 to the eluent generator. This provides a means of makingelectrical contact with the electrodes that is at the same time easy toseal against liquid leakage.

The line X-X is illustrated across the resin bed 126. For reasons whichwill be explained below, the resin upstream of the dotted line may bepredominantly or completely in the form of the cation counter ion of thebase used as the electrolyte during separation. Downstream of the lineX-X, the resin may be predominantly or completely in the hydronium form.The line X-X represents the interface.

For anion analysis, a polarizing DC potential is applied between cathode142 and anode 136, and the following reactions take place.

The water is electrolyzed and hydronium ions are generated at anode 136according to the following reaction:H₂O-2e ⁻→2H⁺+½O₂↑  (4)

This causes cations in the cation exchange resin bed 126 to migrate tobarrier 140. This, in turn, displaces hydronium ions upwardly throughbed 126 which causes a similar displacement of cations ahead of them.The cations electromigrate toward the barrier 140 to be transportedacross the barrier 140 toward cathode 142 in cathode chamber 144 whilewater is electrolyzed at cathode 142 to generate hydroxide ionsaccording to the following reaction:2H₂O+2e ⁻→2OH⁻+H₂↑  (5)

The cations which have transported across the barrier combine with thegenerated hydroxide ions to form cation hydroxide in cathode chamber144. The effluent from separator bed percolates through the cation formresin in inlet bed section 126 until it reaches the hydronium form resinin bed section 126 where it is neutralized while the cation is retainedon the resin. At this point, the anion salts are converted to theirrespective acids and the cation hydroxide is converted to weakly ionizedform, water.

The suppressed effluent liquid containing the separated anions leavesbed 126 through conduit 130 and passes to conductivity cell 132 in whichthe conductivity of the separated anions is detected.

The suppressor of FIG. 2 has been described with respect to a system forthe analysis of anions. However, the system is also applicable to theanalysis of cations. In this instance, electrode 136 is a cathode andelectrode 142 is an anode. The ion exchange type of resin is reversed.Thus, the resin in separator bed 118 is a cation exchange resin and theresin in suppressor bed 126 is an anion exchange resin. The plug ormembrane 140 is made of an anion exchange material.

Briefly described, the suppressor works as follows for the cationanalysis. The aqueous liquid stream containing cations to be detectedand an acid electrolyte aqueous eluent are directed through separatorbed 118 including cation exchange resin. The effluent from separator bed118 flows through suppressor bed 126 including anion exchange resin withexchangeable hydroxide ions. The acid in the eluent is converted toweakly ionized form. Some of the exchangeable hydroxide is displaced byanions from the acid.

An electrical potential is applied between the cathode 136 and anode142. Water is electrolyzed at electrode 136 to generate hydroxide tocause anions on the anion exchange resin bed to electromigrate towardbarrier 140 to be transported across the barrier toward the positivelycharged anode 142 in the ion receiving flow channel in electrode chamber144 while water in chamber 144 is electrolyzed to generate hydroniumions which combine with the transported anions to form acid in theelectrode chamber 144. The effluent liquid from the suppressor bed 126flows past detector 132 in which separated cations are detected and isrecycled to electrode chamber 144.

Referring again to FIG. 2, one embodiment of the eluent generator isillustrated, describing first the system for anion analysis in which abase generated in electrode chamber 144 is directed to the eluentgenerator. This embodiment is analogous in electrochemical operation tosuppressor 124. In this embodiment of the eluent generator, a suitablehousing 154 contains an electrolyte ion reservoir in the form of apacked bed of ion exchange resin 156. Resin bed 156 is separated from afirst generator electrode chamber 158 by a charged generator barrier 160which prevents significant liquid flow but permits transport ofelectrolyte ions and thus may be of the type described with respect tosuppressor barrier 140. A generator electrode 162 is disposed andenclosed in generator electrode chamber 158 and may be of the same typeof construction as electrode chamber 144. At the opposite side ofbarrier 160 from electrode 162 is flow-through generator electrode 164analogous in function and structure to suppressor electrode 136.

The electrochemical reactions described above with respect to thesuppressor occur in the eluent generator and so are incorporated hereinby reference. Thus, for analysis of anions, the line x-x separates theinlet section 156 a from the outlet section 156 b of resin bed 156. Thefeed stream in line 152 flows into inlet section 156 a in the cationform while the outlet section is in the hydronium ion form. However, onedifference is that the feed stream in conduit 152 already includes base.The feed stream exits packed resin bed 156 adjacent barrier 160 andflows across bed 156 and out the outlet through electrode 164 or pastsome other form of electrode as described above. Similarly, the packedbed includes resin in the electrolyte ion form (e.g., potassium orsodium) at its inlet end adjacent barrier 160 and in hydrogen ion formnear the outlet end adjacent electrode 164.

The same type of packed bed resin or other form of matrix may be used inthe eluent generator as in the suppressor. As illustrated, the source ofaqueous liquid flowing through generator electrode chamber 158 can beliquid recycled from the outlet of the resin bed chamber 154.Specifically, such liquid flows through conduit 165 and into a mixed(cation and anion exchange) bed water polishing column, or eluentpurification column 32. This column is typically 2 to 40 cm in lengthand 0.5 to 10 cm in internal diameter. From there, the stream flowsthrough conduit and flows to eluent reservoir 18. In the case of anionanalysis, the cation hydroxide is generated in chamber 58 adjacent thecathode in the manner described above with respect to the suppressor.The water source after passing through the water polisher is deionizedand so does not interfere with the analysis. An optional eluentpurification column 44 may be placed after pump 20 to further purify thedeionized water stream

A chromatography system may generate non-stoichiometric quantities of H₂or O₂ for full catalytic conversion in the catalytic gas eliminationcolumn. In another embodiment of the invention as illustrated in FIG. 3,a gas generation device, preferably an electrolytic one, may be placedon line to adjust the gases to stoichiometric amounts, therebyeliminating most or, preferably, substantially all of the hydrogen andoxygen gases in the eluent using the catalytic gas elimination column.

The system shown in FIG. 3 is illustrated using a methanesulfonic acid(MSA) eluent generator. It produces oxygen gas during the process ofgenerating the MSA eluent. Oxygen gas in the MSA eluent stream leavingthe eluent generator can be removed by passing the stream through apiece of permeable tubing of the type used in Dionex EG50 eluentgenerator degas module (Dionex Corporation, Sunnyvale, Calif. U.S.A.).In the system shown in FIG. 3, the stream of the MSA solution containingoxygen gas is passed through an electrolytic gas generator that is usedto generate a stoichiometric amount of hydrogen gas relative to theamount of oxygen in the MSA eluent stream. The stream exiting theelectrolytic gas generator is then passed through the catalytic gaselimination column where the stoichiometric amounts of hydrogen andoxygen gases react catalytically to form water. The MSA eluent streamleaving the catalytic gas elimination column is therefore substantiallyfree of gas and may be used as the eluent in the ion chromatographysystem such as the one illustrated in FIG. 3.

Referring specifically to FIG. 3, like parts with FIGS. 1 and 2 aredesignated with like numbers. A carrier solution (e.g., deionized water)from the eluent reservoir 18 in the container 38 is directed via a pump20 in line 42 through an eluent purification column 44 into the eluentgeneration chamber 52 of the electrolytic MSA eluent generator 50.

The construction of the electrolytic MSA generator can be similar to thetype illustrated in FIG. 1 of the U.S. Pat. No. 6,225,129. The apparatusincludes MSA ion source reservoir 50 a that can be filled with asolution of MSA electrolyte. The MSA eluent generation chamber 52 isseparated from the MSA ion source reservoir 50 a by a barrier 56,suitably in the form of a charged perm-selective membrane describedbelow. Barrier 56 substantially prevents liquid flow while providing anion transport bridge for MSA ions from the ion source reservoir 50 a tothe MSA generation chamber 52. As used herein, the term “barrier” refersto the charged material (e.g. membrane) separating reservoir 50 a andchamber 52 which permits ion flow but blocks substantial liquid flow,alone or in combination with an appropriate flow-through housing inwhich the barrier is mounted transverse to flow across the entire flowpath. The charged barrier 56 should be of sufficient thickness towithstand the pressures in chamber 52. For example, if chamber 52 is online with a chromatography system, such pressures may be on the order of1,000 to 3,000 psi.

A cathode 58 is disposed in electrical contact with, and preferablywithin, MSA ion source reservoir 50 a and an anode 54 is disposed inelectrical contact with, and preferably within, base generation chamber52. A suitable DC power supply (not shown) connects the anode and thecathode. Also, there is a continuous electrical path from anode 54through barrier 56 to cathode 58.

For the production of pure acid (e.g. MSA), high-purity deionized waterfrom the eluent purification column 44 is directed into to generationchamber 52. Water splitting takes place at both electrodes. The cathodereaction in reservoir 50 a is as follows:2H₂O+2e ⁻→2OH⁻+H₂↑  (6)

During this reaction, the hydroxide ions are produced by the cathode inreservoir 50 a. In one form of reservoir 50 a, the ion source is aMSA-containing solution, suitably a methanesulfonic acid solution. Inthis manner, hydroxide ions react with hydronium ions to form water,methanesulfonate anion is the primary ion passing through barrier 56,thereby minimizing the flow of hydroxide ions. The reaction of hydroniumions and hydroxide ions in the reservoir provides electrical neutralityto the solution in the reservoir as the MSA ions are driven across thebarrier 56 under the applied electrical field into the generationchamber 52 where hydronium ions are produced in the following anodicreaction.H₂O-2e ⁻→2H⁺+½O₂↑  (7)

The combination of MSA ions and hydronium ions leads to production ofacid (MSA) in the flowing aqueous stream. The concentration of MSAformed is directly proportional to the applied current and inverselyproportional the flow rate.

The flowing aqueous solution from the eluent generation chamber containsMSA and oxygen gas. This mixture is then directed via line 46 into theelectrolytic gas generator 50. The electrolytic gas generator 50 may beformed of a column 60 packed with a cation ion exchange bed 60 a.Electrodes, in a form to be described below, are spaced apart in theelectrolytic gas generator, with one electrode separated from the resinby a barrier described below. The electrodes are connected to a directcurrent power supply (not shown). The configuration is such that with anaqueous stream flowing through the electrolytic gas generator and theapplication of power, water in the aqueous stream is electrolyzed toform a source of hydrogen gas and hydroxide ions at the cathode andoxygen gas and hydronium ions at the anode.

Because of its ready availability and known characteristics, a preferredform of ion exchange bed 60 a is a packed ion exchange bed of resinparticles. It is desirable that the resin particles be tightly packed inthe bed, to form a continuous ion bridge or pathway for the flow of ionsbetween electrodes 62 and 66. Also, there must be sufficient spacing forthe aqueous stream to flow through the bed without undue pressure drops.

The resin is suitably contained in the column by a porous frit whichserves to provide an outlet to the column. In the illustratedembodiment, the porous frit is porous electrode 62 which serves the dualfunction of containment of the resin and as an electrode. Cathode 62disposed at the outlet end of resin bed 60 a preferably is in intimatecontact with the resin therein.

A barrier 64 separates resin bed 62 from electrode 66 (anode) in theinterior of a hollow housing defining electrode chamber 68 preventingany significant liquid flow but permitting transport of ions only of thesame charge as the charge of exchangeable ions on resin bed 26. Forgeneration of hydrogen gas at cathode 62, barrier 64 is suitably in theform of a cation exchange membrane or plug separating electrode chamber68 from the cation exchange resin.

Electrode 66 in electrode chamber 68 also suitably is in the form of aninert metal (e.g., platinum) porous electrode in intimate contact withbarrier 64. An electrode is fabricated in a way to permit goodirrigation of the electrode/membrane interface when water is passedthrough electrode chamber 68. The electrode is suitably prepared bycrumpling and forming a length of fine platinum wire so as to produce aroughly disc-shaped object that allows easy liquid flow throughout itsstructure and at the electrode membrane interface. Good contact betweenthe disc-electrode 66 and barrier 64 is maintained simply by arrangingthat the one press against the other. The electrode can extend acrossall or part of the aqueous liquid flow path through electrode chamber 68to provided intimate contact with the flowing aqueous stream. A conduit70 is provided to direct the aqueous liquid stream to the inlet ofelectrode chamber 68. Conduit 72 takes the effluent from chamber 68 towaste.

By passing a DC current between anode 66 and cathode 62 of theelectrolytic gas generator 60, a controlled amount of hydrogen gas canbe generated at the device cathode 62. The amount of hydrogen gasgenerated is directly proportional to the applied current. Depending onthe amount of oxygen gas in the incoming stream of MSA eluent, asubstantially stoichiometric amount of hydrogen gas can be generated bycontrolling the applied current to the electrolytic gas generator. Inthis manner, the aqueous stream of MSA solution flowing out of theelectrolytic gas generator 60 contains the substantially stoichiometricamounts of hydrogen and oxygen gases. This stream is directed into thecatalytic gas elimination column 31 where hydrogen and oxygen gasesreact catalytically to form water. The MSA eluent stream leaving thecatalytic gas elimination column is, therefore, substantially free ofgas and used as the eluent in the ion chromatography process involvingother down stream components.

Arranged in series with sample injector 16 and separation column 10 is asuppressor 12 serving to suppress the conductivity of the electrolyte ofthe eluent from column 10 but not the conductivity of the separatedions. The effluent from suppressor 12 is directed to a detector,preferably in the form of a detector flow-through conductivity cell 14,for detecting the ion species resolved in column 10.

In one embodiment, the effluent from conductivity detector 14, referredto as the detector effluent, is directed in a recycle conduit 22 to atleast one flow-through detector effluent flow channel 24 in suppressor12. An ion exchange membrane 26 separates detector effluent flow channel24 from chromatographic separation effluent flow channel 28 whichreceives the effluent from chromatography column 10. In the simplifiedversion illustrated, only a single detector effluent flow channel 24 isused. The system of the present invention is also applicable to othermembrane suppressors such as the sandwich suppressor type illustrated inU.S. Pat. No. 5,248,426. In a sandwich suppressor, the chromatographicseparation effluent flows through a central flow channel flanked by twodetector effluent flow channels separated by ion exchange membranes. Inthis embodiment, the detector effluent flow channels may be suppliedwith the detector effluent from conductivity detector 14 by use of asplitter valve. The details of such a sandwich suppressor and the use ofrecycle from the conductivity cell are supplied by the detector effluentflow channel as illustrated in U.S. Pat. No. 5,248,426.

As illustrated in FIG. 3, the effluent from the suppressor effluent flowchannel 24 is directed via line 30 into a cation trap column 32 which ispacked with a cation exchange resin in hydronium form. The cation trapcolumn 32 is used to remove contaminants of sample injected throughinjection valve 16, and other trace contaminants may be generated fromoperation of the ion chromatography system.

The system of FIG. 3 is also applicable to the generation of a baseeluent with appropriate reversal of polarity of the reagents and chargedcomponents for anion analysis.

In the above embodiments of the present invention, water formed by thecatalytic combination of hydrogen and oxygen gas can serve as a sourceof water for the eluent stream by flowing the produced water into theeluent stream. The gases can be electrolytically generated in thechromatography system or supplied from independent sources of hydrogenand oxygen gas such as from pressurized containers.

The following examples demonstrate the present invention for systems inwhich the eluents are prepared off-line and recycled and water is usedin the electrolytic eluent preparation in ion chromatography.

EXAMPLE 1 Ion Chromatographic Separation of Common Anions Using anElectrolytic Suppressor and Recycle of a Sodium Carbonate/SodiumBicarbonate Eluent

This example illustrates the use of the eluent-recycle ionchromatography system shown in FIG. 1 for determination of common anionsincluding chloride, nitrate, and sulfate. A Dionex® ICS-1500 ionchromatography system consisting of an isocratic dual-piston highpressure pump, a six-port injector, a column oven, and a conductivitydetector was used. A Dionex 4-mm AS4A SC column was used as theseparation column, a solution of 3 mM sodium carbonate was used as theeluent, and the separation was performed at 1.5 mL/min. A Dionex anionAtlas® electrolytic suppressor was used in the experiments. Thecatalytic gas elimination column was prepared by packing small strips ofporous Pt foil in a ⅛ OD× 1/16 ID×40 cm plastic column. The eluentpurification column was packed with ion exchange resins as describedpreviously. In one set of experiments, a sample solution containingchloride, nitrate and sulfate was injected daily. The retention time andchromatographic efficiency of each analyte were monitored over a periodof 500 hours during which a 2-liter solution of 3 mM sodium carbonatewas recycled continuously in the system setup shown in FIG. 1.

Reproducibility data was plotted for the retention time obtained forchloride over a period of 500 hours of continuous operation. The averageretention time determined for chloride was 1.202 minutes and the percentRSD (relative standard deviation) of chloride retention time was lessthan 0.5%. During the same period of time, the average retention timeswere 1.822 minutes for nitrate and 3.44 minutes for sulfate, and thepercent RDS of retention times of nitrate and sulfate were less than0.6% and 2.7% respectively

These results indicate that the ion chromatography system shown in FIG.1 can be used to perform reproducible separation of analyte ions ofinterest using recycled sodium carbonate eluent over an extended periodof time. The operation of an ion chromatography system in such a formatsimplifies the system operations, minimize waste disposal, and reduceoperating costs.

EXAMPLE 2 Ion Chromatographic Separation of Common Cations Using anElectrolytic Suppressor and Recycle of a Sulfuric Acid Eluent

This example illustrates the use of the eluent-recycle ionchromatography system shown in FIG. 1 for determination of commoncations including lithium, sodium, ammonium, potassium, magnesium, andcalcium. A Dionex DX500 ion chromatography system consisting of adual-piston high pressure pump, a six-port injector, a column oven, anda conductivity detector was used. A Dionex 4-mm CS12A column was used asthe separation column, a solution of 22 mN sulfuric acid was used as theeluent, and the separation was performed at 1.0 mL/min. A Dionex cationAtlas® electrolytic suppressor was used in the experiments. In thisexample, the catalytic gas elimination column and the eluentpurification column were not used. A Dionex TMC-1 trace metalconcentrator column was used as the cation trap column placed betweenthe outlet of the conductive cell and inlet of the suppressor regenerantchamber. In one set of experiments, a sample solution containinglithium, sodium, ammonium, potassium, magnesium, and calcium wasinjected daily, the retention time and chromatographic efficiency ofeach analyte were determined to be stable over a period of 14 daysduring which a 1400-mL solution of 22 mN sulfuric acid was recycledcontinuously. During the period of 14 days, the relative standarddeviation of retention time of each analyte was less than 1.1%, and therelative standard deviation of chromatographic efficiency of eachanalyte was less than 2.3%.

EXAMPLE 3 Ion Chromatographic Separation of Common Cations Using anElectrolytic Suppressor and Recycle of an Eluent ContainingMethanesulfonic Acid and 2-butanone (MEK, Methyl Ethyl Ketone)

This example also illustrates the use of the eluent-recycle ionchromatography system shown in FIG. 1 for determination of commoncations including lithium, sodium, ammonium, potassium, magnesium, andcalcium. In this example, a Dionex DX500 ion chromatography systemconsisting of a dual-piston high pressure pump, a six-port injector, acolumn oven, and a conductivity detector was used. A Dionex 4-mm CS15column was used as the separation column, a solution of 12 mMmethoanesulfonic acid (MSA) and 5% (v/v) HPLC-grade butanone was used asthe eluent, and the separation was performed at 1.0 mL/min. A Dionexcation Atlas® electrolytic suppressor was used in the experiments. Noeluent purification or catalytic gas elimination columns were used. ADionex TMC-1 trace metal concentrator column was used as the cation trapcolumn placed between the outlet of the conductive cell and inlet of thesuppressor regenerant chamber. In this set of experiments, a samplesolution containing chloride, nitrate and sulfate was injected daily,the retention time and peak area response of each analyte weredetermined to be stable over a period of 25 days during which a 1400-mLsolution of 12 mM methoanesulfonic acid (MSA) and 5% (v/v) HPLC-gradebutanone was recycled continuously. During the period of 24 days, therelative standard deviation of retention time of each analyte was lessthan 0.6%, and the relative standard deviation of chromatographicefficiency of each analyte was less than 2.0%.

The ion chromatography system shown in FIG. 1 can be used to performreproducible separation of cations of interest using an ionchromatographic eluent containing an organic solvent that is recycledover an extended period of time. The operation of an ion chromatographysystem in such a format simplifies the system operations, minimize wastedisposal, and reduce operating costs.

EXAMPLE 4 Ion Chromatographic Separation of Common Anions Using anIon-Reflux Based Ion Chromatography System with the Recycle of WaterCarrier Stream

The major system components used in this example are shown in FIG. 2. ADX500 ion chromatographic system (Dionex Corporation, Sunnyvale, Calif.)consisting of a GP40 pump and an AS15 separator column (4-mmID×250-length) was used. A Dionex ASRS anion suppressor (P/N 53946) wasused as the suppressor. A built-in power supply in a Dionex ED40detector was used to supply 300 mA of DC current the suppressor. ADionex ED40 conductivity detector equipped with a flow-throughconductivity cell was used to monitor the effluent from the suppressor.The eluent generation and recycle module was similar to one described inour previous studies as described in U.S. Pat. No. 6,562,628). A DionexEG40 Eluent Generator Module was used to supply the DC current to theanode and cathode of the eluent generation and recycle module. A DionexPeakNet 5.0 computer workstation was used for instrument control, datacollection, and processing.

In one set of experiment, a 4-liter source of deionized water was usedas the carrier stream, the eluent generator and recycle module was usedto generate a hydroxide gradient of 1 to 100 mN KOH at 1.0 mL/min, theseparation of seven common anions on the AS15 column was monitoredcontinuously over a period of 44 days, and the water leaving the anodechamber of the eluent generator and recycle module was recycled. FIG. 4compares the chromatograms obtained for the separation of fluoride,chloride, carbonate, sulfate, nitrate, and phosphate on the AS15 columnusing (1) the deionized water that was used continuously for 44 days and(2) a fresh source of deionized water. The two chromatograms overlayperfectly in terms of analyte retention time. The background conductanceof the chromatogram obtained using the recycled water remained very lowusing the recycled water (only slightly higher than that obtained usingthe fresh deionized water). The conductance of the recycled water, whichwas also monitored continuously over the period of 44 days. The resultsindicate that the conductance of recycled water remained essentiallyunchanged over the testing period after the initial increase to 2 μSover the first 24 hours of the recycling due to the absorption of carbondioxide from the air.

These results indicate that the ion chromatography system shown in FIG.2 can be used to perform reproducible separation of cations of interestusing water that is recycled over an extended period of time. Theoperation of an ion chromatography system in such a format simplifiesthe system operations, minimize waste disposal, and reduce operatingcosts.

EXAMPLE 5 Ion Chromatographic Separation of Common Cations Using an IonChromatography System Equipped with an Electrolytic Gas GenerationDevice and a Catalytic Gas Elimination Column

This example illustrates the use of the ion chromatography system shownin FIG. 3 for determination of common cations including lithium, sodium,ammonium, potassium, magnesium, and calcium. A Dionex ICS-2000 ionchromatography system consisting of an isocratic dual-piston highpressure pump, a six-port injector, a column oven, and a conductivitydetector was used. A Dionex electrolytic pH modifier converted to thehydronium form was used as the electrolytic gas generator. The catalyticgas elimination column was prepared by packing small strips of Pt foilin a 4-mm ID×50-mm PEEK column. A Dionex 4-mm CS12A column was used asthe separation column. A Dionex CSRS Ultra II suppressor was used in theexperiments. FIG. 5 shows the separation of six common cations obtainedusing an ion chromatography system assembled according to FIG. 3. Theseparation of six cations on the CS12A column was achieved using aneluent of 20 mM MSA generated using a Dionex EGG MSA cartridge at 1.0mL/min.

In ion chromatography system employing the methanesulfonic acid (MSA)eluent generator, oxygen gas is produced during the process ofgenerating the MSA eluent. Oxygen gas in the MSA eluent stream leavingthe eluent generator is removed typically by passing the stream througha piece of permeable tubing of the type used in Dionex EG50 eluentgenerator degas module (Dionex Corporation, Sunnyvale, Calif. U.S.A.).In the example, the stream of the 20-mM MSA solution containing oxygengas was passed through the electrolytic gas generator. A DC current of32 mA was supplied to the electrolytic gas generator to produce astoichiometric amount of hydrogen gas relative to the amount of oxygenin the MSA eluent stream. The stream exiting the electrolytic gasgenerator was then passed through the catalytic gas elimination columnwhere the stoichiometric amounts of hydrogen and oxygen gases reactedcatalytically to form water. The MSA eluent stream leaving the catalyticgas elimination column was therefore free of gas and used as the eluentin the ion chromatography process involving other down streamcomponents. The results shown in FIG. 5 indicate that the ionchromatography system shown in FIG. 3 can be used to perform ionchromatographic separation of target analyte ions.

1. A chromatographic method comprising: (a) chromatographicallyseparating sample ionic species in an aqueous liquid eluent streamflowing through a chromatography separation medium, to form achromatography effluent, (b) suppressing the chromatography effluentfrom step (a) by flowing it through a chromatography effluent flowchannel on one side of a first ion exchange membrane in a membranesuppressor, (c) flowing suppressor effluent from said chromatographyeffluent flow channel past a flow-through detector to form a detectoreffluent stream, (d) recycling the detector effluent from step (c) to adetector effluent flow channel in said membrane suppressor on theopposite side of said first membrane from said chromatography effluentflow channel, (e) mixing effluent from said detector effluent flowchannel with a source of eluent and flowing the mixture to saidchromatography separation medium, and (f) catalytically combininghydrogen and oxygen gas, or catalytically decomposing hydrogen peroxide,or both, in said detector effluent stream in a catalytic gas eliminationchamber to form water thereby reducing the gas content in said detectoreffluent stream.
 2. The method of claim 1 further comprising the stepof: (f) at least partially removing ionic components in said detectoreffluent stream prior to mixing with said eluent source.
 3. The methodof claim 1 further comprising the step of: mixing said detector effluentstream with eluent from an eluent source prior to flowing through saidseparation medium.